1. Field of the Invention
The present invention relates to an inkjet head for jetting ink by applying pressure to a pressure chamber, and more particularly, to an inkjet head for improving the speed of ink replenishment into the pressure chamber.
2. Description of the Related Art
Inkjet heads perform recording by jetting ink drops onto a recording medium, and are widely used in small-scale printers. FIG. 21 and FIG. 22 are compositional diagrams of a conventional inkjet head. This head 10 is an example wherein a bimorph actuator is adopted, comprising a piezoelectric element 101 layered on a vibrating plate 102 to form a driving element.
In this head 10, ink is supplied to the head 10 from an ink tank 108, and moreover, inside the head 10, ink is supplied to respective pressure chambers 104 and nozzles 106 by means of a common ink passage 107 and ink supply passages 110.
When a drive signal from a drive circuit is supplied to the individual electrode 100 on the piezoelectric element 101 (there being one electrode corresponding to each nozzle), the vibrating plate 102 is caused to distort towards the inside of the pressure chamber 104, by the piezoelectric effect of the piezoelectric element 101, thereby causing ink to be expelled from the nozzle 106. This ink forms a dot on the printing medium, whereby a desired image is created.
In this way, in a multi-nozzle head 10, each pressure chamber 104 is connected to the common ink passage (chamber) 107 by means of respective ink supply passages 110, and ink is supplied to each pressure chamber 104 from the common ink passage 107. Therefore, after ink has been expelled from the nozzle 106, ink is replenished from the common ink chamber 107 to the pressure chamber 104, via the ink supply passage 110. An acoustic capacitor section 109 is provided in the common ink passage 107, in order to absorb and alleviate pressure fluctuations in the respective pressure chambers 104.
An acoustic equivalent circuit of the head 2 having this composition is illustrated in FIG. 23. It can be approximated to the equivalent circuit shown in FIG. 24 when ink is expelled, and to the equivalent circuit shown in FIG. 25 when ink is being replenished. In other words, when jetting ink, an acoustic capacitance C2 of the piezoelectric element 101 and the pressure chamber 104 is added to the circuit, because the pressure chamber 104 is operated by the piezoelectric element 101. While, the process of replenishing ink is expressed by an LCR second-order delay system comprising the acoustic resistance R1 and inductance L1 between the nozzle 106 and supply passage 110, and the acoustic capacitance of the meniscus of the nozzle.
Since the acoustic capacitance of the meniscus, c1, is an order of 10 greater than the acoustic capacitance between the piezoelectric element and pressure chamber, the intrinsic frequency of the ink emission is several 10 to 100 and several 10 KHZ, which permits high-speed operation in the 10 microsecond order, whereas the intrinsic frequency of the replenishment of the injected ink is several to several 10 KHz, which allows 100 microsecond-order operation. Therefore, the operating frequency of the inkjet head is limited by the replenishment frequency, and hence it is difficult to increase the operating frequency beyond several 10 KHz.
This situation is made evident by the frequency characteristics of the charge flow in the approximate equivalent circuit of FIG. 25, which are illustrated in FIG. 26. More specifically, taking xcfx890 as the intrinsic frequency of the LCR second-order delay in FIG. 25, xcfx89/xcfx890 is plotted on the horizontal axis, and the normalized charge flow is illustrated on the vertical axis. In the case of an inkjet head, the charge flow corresponds to the volume displacement of the ink. The parameter damping factor, xcex4, is expressed by the following equation.
xcex4=0.5*(Rn+Rc+Rs)*(Cn/(Ln+Lc+Ls)) 
Consequently, at a damping factor of xcex4=0.5, which is the normal optimum value, if xcfx89/xcfx890 exceeds xe2x80x9c1xe2x80x9d, then the charge flow declines sharply, in other words, the ink volume displacement reduces, and therefore, using the head at a frequency at or above the intrinsic frequency xcfx890 will lead to ink supply shortage, and hence the volume of ink injected will decrease sharply. For example, in the case of a fabricated head having nozzle diameter of 20 microns, which injects 2.0 pl ink particles, the intrinsic frequency of the ink emission will be 111.9 KHZ, whereas the intrinsic frequency of the ink replenishment will be 34.5 KHz, this latter intrinsic frequency being some 3.2 times greater than the emission frequency. This means that it is difficult to perform high-speed printing which makes full use of the ink emission capacity. Moreover, when performing high-speed printing, it is necessary to increase the number of nozzles for injecting ink (for example, to several hundred or more), and this leads to increased device costs.
It is an object of the present invention to provide an inkjet head for enabling high-speed printing, by increasing the operating frequency of ink replenishment.
It is a further object of the present invention to provide an inkjet head for achieving high-speed printing in an inexpensive manner, by increasing ink replenishment speed, without using costly functional elements, and the like.
It is yet a further object of the present invention to provide an inkjet head for increasing the operating frequency of ink replenishment by means of a simple composition.
In order to achieve these objects, the inkjet head according to the present invention comprises: pressure chambers communicating with the nozzles; energy generating sections for imparting ink injection energy to the pressure chambers; and supply passages for supplying ink from an ink chamber to the pressure chambers; each of the supply passages comprising: an independent damper; a first ink supply passage connecting the independent damper with the pressure chamber; and a second ink supply passage connecting the independent damper with the ink chamber.
In the present invention, the independent damper is provided in the ink supply passage to impart an acoustic capacitance to the ink supply passage. Therefore, a reciprocal action is generated between the pressure chamber and the ink supply passage, whereby the ink replenishment operation can be stimulated. Consequently, the frequency of ink replenishment can be expanded by a factor of approximately 3, and the ink emission can be increased accordingly, without having to provide functional element in the ink supply passage.
Moreover, in the inkjet head according to the present invention, if the independent damper has an acoustic capacitance approximately equal to that of the meniscus of the nozzle, then the ink replenishment frequency can be increased due to the composition of the independent damper.
Furthermore, in the inkjet head according to the present invention, this can be achieved readily by setting the acoustic capacitance Cd of the independent damper within a range of 1 to 2 times the minimum value Cn of the acoustic capacitance of the meniscus.
Moreover, in the inkjet head according to the present invention, by constituting the nozzles in such a manner that the scale of change in the acoustic capacitance of the meniscus of the nozzle is less than a factor of 2, any change in the acoustic capacitance of the nozzles can be prevented from affecting the ink replenishment frequency.
Furthermore, in the inkjet head according to the present invention, by adopting a composition wherein the sectional area of the first ink supply passage is smaller than the sectional area of the pressure chamber and the independent damper, and the sectional area of the second ink supply passage is smaller than the sectional area of the ink chamber and the independent damper, it is possible to exclude the effects of the fluid resistance of the ink.
Moreover, in the inkjet head according to the present invention, when the independent damper has a structure wherein the ink chamber is covered by an elastic member, then a large acoustic capacitance can be achieved whilst maintaining a small chamber, and hence increase in the head size can be prevented.
Furthermore, in the inkjet head according to the present invention, by constituting the independent damper by means of a meniscus forming section, the acoustic capacitance of the nozzle meniscus can be achieved readily in the ink supply passage.
Moreover, in the inkjet head according to the present invention, when the energy generating section comprises a piezoelectric element, then the high-speed characteristics of a piezoelectric head can be utilized.
Further objects and modes of the present invention will become apparent from the following description of the embodiments and accompanying drawings.